In oilfield service work, a piece of equipment referred to in the trade as a "tree saver" or wellhead isolation tool is often used. This apparatus generally introduces a high pressure mandrel with a sealing nipple through the low pressure rated valves and fittings on a wellhead and the resilient sealing material on the nipple at the lower end of the mandrel seals the mandrel in the well tubing or casing. This allows high pressure fluids to be introduced into the well tubing or casing through the mandrel without their having any communication with these low pressure rated valves and fittings. The state of the art equipment which utilizes concentric mandrels and an expanding nipple is that described both in my Canadian Pat. No. 1,217,128 issued Jan. 27, 1987 and my U.S. Pat. No. 4,657,075, issued Apr. 14, 1987. A later configuration of the tool, having improvements over the original, is now described. It utilizes the principle of the concentric mandrels and the sealing nipple and expander.